1. Field of the Invention
The present invention relates to a reflective liquid crystal display device and a liquid crystal display device operable both in a reflection mode and a transmission mode, and which are used for office automation equipment such as wordprocessors and personal computers, mobile information devices such as hand-held computers, and VTRs integrated with a camera and having a liquid crystal monitor. The present invention also relates to a method for producing such liquid crystal display devices. In this specification, a liquid crystal display device will be referred to as an “LCD device”. A liquid crystal display device operable both in a reflection mode and a transmission mode will be referred to as a “transmission- and reflection-type LCD device”.
2. Description of the Related Art
LCD devices do not emit light themselves unlike CRTs (cathode ray tubes) and EL (electroluminescence) devices. Accordingly, transmissive LCD devices equipped with a backlight on a rear surface thereof are used.
The backlight usually consumes 50% or more of the total power consumption of the LCD device. Some mobile information devices which are often used outdoors or constantly carried by the user include a reflective LCD device which includes a reflective plate and performs display using only the ambient light.
Reflective LCD devices include TN (twisted nematic) mode devices and STN (super twisted nematic) mode devices which use a polarizer and are in a wide use as transmissive LCD devices today, as well as phase change (PC) guest-host mode devices which have been actively developed recently. The PC guest-host mode devices do not use a polarizer and thus realize brighter display. Such a device is disclosed in, for example, Japanese Laid-Open Publication No. 4-75022 corresponding to U.S. Pat. No. 5,220,444 and Japanese Laid-Open Publication No. 9-133930.
However, the PC guest-host mode LCD devices perform display using optical absorption by dyes in a liquid crystal layer including liquid crystal molecules and the dyes dispersed therein. Accordingly, the phase transition guest-host mode LCD devices provide significantly lower quality than the TN devices and the STN devices using a polarizer.
In LCD devices including the liquid crystal molecules aligned in parallel or in a twisted manner, the liquid crystal molecules at the center and in the vicinity of the liquid crystal layer tilt vertically to surfaces of substrates. However, the liquid crystal molecules in the vicinity of alignment layers do not tilt vertically to the surfaces of the substrates. Accordingly, the birefringence of the liquid crystal layer cannot be 0. Therefore, in the case where the LCD device operates in a display mode for performing black display when a voltage is applied, satisfactory black display is not performed due to the remaining birefringence. Thus, sufficient contrast ratio is not obtained.
The TN mode and STN mode devices do not provide sufficiently high quality display in terms of brightness and contrast. Accordingly, further improvement in the brightness and the contrast is demanded.
Reflective LCD devices are disadvantageous in that the intensity of the reflected light used for display is lowered when the ambient light is dark. By contrast, transmissive LCD devices are disadvantageous in that the visibility is lowered when the ambient light is very bright, for example, outdoors on a fine day.